igneous
Polished basalt pebble
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark brown-black, fine-grained, subrounded and water-worn, with a dull to slightly glassy luster and scattered vesicles or weathered pits. Hardness is about 5–6 Mohs; cleavage is not apparent and specific gravity is typically 2.8–3.0.
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Physical properties
Dark brown-black, fine-grained, subrounded and water-worn, with a dull to slightly glassy luster and scattered vesicles or weathered pits. Hardness is about 5–6 Mohs; cleavage is not apparent and specific gravity is typically 2.8–3.0.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface; basalt ranges from Precambrian to modern age and is abundant in oceanic crust and volcanic terrains.
Uses & applications
Used as aggregate, roadstone, building stone, and occasionally as a decorative or lapidary pebble; this small specimen has modest collectible value.
Geological facts
The brown coloration likely reflects iron-oxide weathering, while the small cavities may be former gas bubbles or weathered mineral grains.
Field identification & locations
Identify basalt by its heavy feel, dark fine-grained texture, lack of visible large crystals, and weak attraction to a magnet if iron-rich. Common sources include Hawaii, Iceland, ocean beaches, river gravels, and volcanic regions.
